1.Field of the Invention
The present invention relates to a method and an apparatus for manufacturing pneumatic tyres for vehicle wheels.
2. Description of the Related Art
In a tyre production cycle it is provided that, subsequently to a building cycle in which the different tyre components are manufactured and/or assembled, a moulding and vulcanisation process be carried out which is aimed at defining the tyre structure according to a desired geometry, usually having a particular tread pattern.
To this aim, the tyre is closed into a moulding cavity defined internally of a vulcanisation mould and conforming in shape to the geometric configuration of the outer surfaces of the tyre to be obtained.
A pneumatic tyre generally comprises a carcass structure of a ring-shaped toroidal conformation, including one or more carcass plies strengthened with reinforcing cords lying in radial planes, i.e. in planes containing the rotation axis of the tyre. Each carcass ply has its ends rigidly associated with at least one metallic annular anchoring structure, usually known as bead core, constituting the reinforcement to the beads, i.e. to the radially internal ends of said tyre, the function of which is to enable assembling of the tyre with a corresponding mounting rim. A band of elastomeric material referred to as tread band is applied crownwise to said carcass structure, and in said tread band at the end of the vulcanisation and moulding steps a raised pattern is formed for ground contact. Placed between the carcass structure and the tread band is a reinforcing structure currently known as belt structure. This belt structure usually comprises, in the case of tyres for cars, at least two radially superposed strips of rubberised fabric provided with reinforcing cords, usually of metal, disposed parallel to each other in each strip and crossed with the cords of the adjacent strip, preferably symmetrically disposed relative to the equatorial plane of the tyre. Preferably said belt structure, at least on the ends of the underlying strips, further comprises a third layer of textile or metallic cords that are circumferentially disposed (at 0 degrees), at a radially external position.
Respective sidewalls of elastomeric material are also applied to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads.
Finally, in tyres of the tubeless type, i.e. devoid of air tube, there is the presence of a radially internal layer having impermeability features to ensure airtightness to the tyre, said layer being generally referred to as “liner”.
To the aims of the present description it is to be pointed out herein that by the term “elastomeric material” it is intended a composition comprising at least one elastomer polymer and at least one reinforcing filler. Preferably, this composition further comprises additives such as cross-linking agents and/or plasticizers. Due to the presence of the cross-linking agents, this material can be cross-linked through heating, so as to form the final article of manufacture.
There are moulding and vulcanisation methods in which a green tyre is set within the mould, being placed on a substantially rigid toroidal support. Said methods are preferably used for tyres that, based on recent manufacturing processes, are produced starting from a limited number of elementary semi-finished products fed onto a toroidal support the outer profile of which is coincident with that of the radially internal surface of the tyre that is wished to be produced. Said toroidal support is moved, preferably by a robotized system, between a plurality of stations in each of which through automated sequences, a particular tyre manufacturing step is carried out (see document EP 0 928 680 in the name of the same Applicant, for example).
The European Patent Application issued under No. 0 976 533 in the name of the same Applicant discloses a method and an apparatus for moulding and curing tyres for vehicle wheels in which a green tyre manufactured on a toroidal support is enclosed in the moulding cavity of a vulcanisation mould; subsequently, steam or other fluid under pressure is fed into at least one gap for fluid diffusion created between the outer surface of the toroidal support and the inner surface of the tyre, thereby imposing a radial expansion to said tyre that causes pressing of the latter against the inner surfaces of the moulding cavity. Following this pressing operation, suitable forming ridges set in the moulding cavity penetrate into the elastomeric material in the region of the tread band, to generate recesses and grooves such disposed as to form a desired tread pattern.
By a method of the above described type, the cured tyre can however show some faults, because the steam or other working fluid used for vulcanisation comes into direct contact with the innermost layer of the tyre, as for tyres directly assembled and cured on the same toroidal support there is not the effect of the vulcanisation bladder normally used when vulcanisation is carried out on tyres built by assembling semi-finished components without the aid of a toroidal support.
To overcome these drawbacks, the Applicant has already put into practice the above described methods through the teachings of document WO 2004/045837 according to which a green tyre is submitted to a preliminary step of pressing it against the toroidal support while simultaneously administering heat, to obtain at least partial vulcanisation of the innermost layer of the tyre itself and of the bead region. In this manner it is possible to subsequently carry out a moulding and vulcanisation step while expanding the tyre against the outer surfaces of the moulding cavity, without involving lack of homogeneity and of evenness in that tyre portion that is in contact with the toroidal support, which portion is the first to come into contact with the working fluid during vulcanisation. The working fluid used in the moulding and vulcanisation step in fact comes into contact with parts of the tyre that have already been partly cured and in which therefore the behaviour of the material is no longer plastic but almost elastic, thereby withstanding the fluid action without experiencing deterioration or deformations.
The Applicant has however perceived that also in the above described vulcanisation methods submitting the tyre to a preliminary pressing treatment against the toroidal support, surface unevennesses in the radially external parts of the tyre can occur, at least under particular work conditions.
According to the Applicant's perception, in fact, the preliminary pressing step against the toroidal support, aimed at obtaining an at least partial cross-linking of the inner tyre surface, can also cause partial cross-linking of the radially external surface portions of the tyre itself.
In more detail, the Applicant has noticed that closure of the tyre into the moulding cavity can cause the tread band to be partially penetrated by the forming ridges set in the moulding cavity for the purpose of creating the tread pattern so that, as a result, heat transfer and partial cross-linking of the tyre regions directly in contact with the forming ridges occur during the preliminary pressing step. Consequently, unevennesses are created in the elastomeric material constituting the tread band and correct adaptation of the material itself to the conformation of the moulding cavity during the subsequent moulding and vulcanisation step is impaired.
The Applicant has further observed that part of the fluid under pressure, nitrogen for example, admitted into the mould to cause the preliminary pressing treatment against the toroidal support can be entrapped between the outer surface of the tread band and the moulding cavity, in the grooves delimited by the forming ridges to form the pattern blocks on the tyre tread band. Therefore gaseous pockets are created that can be hardly evacuated during the subsequent moulding and vulcanisation step and can cause geometric unevennesses in the finished product.